The present invention relates to a tapered waveguide used for an optical probe in a scanning near-field optical microscope which is one of scanning probe microscopes and can provide information on the characteristics of surface optical properties, a scanning near-field optical microscope using the optical probe having the tapered waveguide, and a method for forming an aperture of the tapered waveguide.
A scanning probe microscope represented by an atomic force microscope (hereinafter referred to as an AFM) or a scanning tunneling microscope (hereinafter referred to as an STM) has wide spread use because of its performance of observing the sample surface in high resolution.
On the other hand, various types of scanning near-field optical microscopes which can provide optical characteristics and topography of sample surface have been proposed. The scanning near-field optical microscopes control the distance between the tip of a sharpened optical probe including an optical waveguide and sample surface to smaller than optical wavelength. One microscope holds the optical fiber probe vertically to the sample and vibrates the tip of the probe horizontally to the sample surface. Variation in vibration amplitude caused by friction between the tip of the probe and the sample surface is detected as a displacement of the optical axis of laser light which has been irradiated from the tip of the probe and transmitted through the sample. The distance between the tip of the probe and the sample surface is kept constant during scanning by controlling a Z-axis positioner. Thus the scanning near-field optical microscope can provide distribution of the intensity of transmitted light through the sample and topography of the sample surface.
Another is a scanning near-field optical/atomic force microscope which uses a sharpened and bent optical fiber probe as a cantilever of an AFM. The scanning near-field optical/atomic force microscope can measure the characteristics of surface optical properties and topography simultaneously by applying a laser light to the sample from the tip of the optical fiber probe during its AFM operation.
Such a scanning near-field optical microscope which measures optical characteristics and a topography of a sample at the same time uses a tapered waveguide for an optical probe. The optical probe has a coating film on its tapered portion except its aperture.
FIG. 7 is a sectional view showing a conventional composition of an optical probe. Number 1 is an optical waveguide whose tip has been sharpened and number 51 is a coating film. The coating film 51 is composed of a single layer and is composed so as to have the same plane as the aperture surface. In case that this optical probe is mounted on a scanning near-field optical microscope, its topographical resolution is limited by the tip diameter of the optical probe including its coating film and its optical resolution by size of the aperture in the probe tip. For example, in case that the tip diameter of the tapered waveguide itself is 100 nm and the thickness of the coating film is 100 nm, provided that the coating film does not enter the aperture, the aperture is 100 nm in diameter and the tip of the optical probe including the coating film is about 300 nm in diameter.
In order to improve the topographical resolution, it is necessary to make the tip of an optical probe small in diameter. However, when the coating film is deposited thin, leaked light through the circumference of an aperture deteriorates optical resolution and contrast of optical characteristics.
On the other hand, when the coating film is deposited thick enough so as to not leak light, optical resolution and contrast of optical characteristics are deteriorated by reduction of an amount of light outputted from the aperture due to the coating film burying of the aperture in addition to deterioration of topographical resolution.
In the composition of the optical probe according to the prior art shown in FIG. 7, for a range of wavelength around 500 nm, a coating film 51 of aluminum can be coated ideally to about 50 nm in thickness, but actually needs to be about 100 nm in consideration of deterioration in its film quality and occurrence of pinholes. In this case, the tip of the optical probe is at least 200 nm or greater in diameter.
Furthermore, in case of additionally depositing a protective film outside the coating film or in case of additionally depositing a functional film such as a magnetic film and the like, the same problem as the above-mentioned case of depositing a thick coating film occurs.